Gray cast iron having improved properties



1949. K. D. MILLIS ET AL 2,485,761

GRAY CAST IRON HAVING IMPROVED PROPERTIES Filed March 20, 1948 2Sheets-Sheet l IN V EN TORS GRAY CAST IRON HAVING IMPROVED PROPERTIESFiled March 20, 1948 2 Sheets-Sheet 2 JNVENTORS BYG'Q'.

197' 7' Off/V5 Patented Oct. 25, 1949 GRAY CAST IRON HAVING IMPROVEDPROPERTIES Keith Dwight Millis, Rahway, Albert Paul Gagnebin, Red Bank,and Norman Boden Pilling, Westfieid, N.'J., assignors to TheInternational Nickel Company, Inc., New York, N. Y a corporation ofDelaware Application March 20, 1948, Serial No. 16,106 In Great BritainMarch 22, 1947 8 Claims.

The present invention relates to an improved gray cast iron andmoreparticularly to a new gray cast iron havingan improved and unusualcombination of properties, including founding properties and mechanicaland physical properties.

- Gray cast iron has been one of the most widely used'ferrousengineering materials. It is easily made, is soft and niachinable andhas physical properties which make it useful in a. wide number ofapplications. Gray cast iron is a ferrous alloy containing graphiticcarbon, 1. e., gray cast iron is a ferrous alloy containing uncombinedcarbon in excess of that carbon required to form the ferrous matrix.This excess uncombined carbon appears in the form of elongated, warped,thin plates or flakes which are disseminated throughout the matrix ofthe iron. Ordinary gray cast iron has been recognized to be a weak,brittle material as compared to materials having compositions andstructures substantially the same as the matrix of the gray cast iron.The comparatively poor mechanical properties possessed by gray cast ironare generally attributed to the effect produced by the presence of theelongated flake graphite particles. Because of the great effect of flakegraphite in lowering the strength, toughness and ductility of gray castiron, it has been found that, when it is attempted to improvetheproperties of gray cast iron through improving the strength ofthe'matrix thereof, large improvements in the strength of the matrixhave produced only relatively minor overall improvements in theproperties of the gray cast iron. Similarly, advances in the melting,founding and processing techniques have produced only relatively smallimprovements in the properties of gray cast iron. Although many attemptshave been made to provides. solution to these and the other problemsrelating to the manufacture of improved gray cast iron, none, as far asis known, has been entirely successful when carried into practice on anindustrial scale.

It has now been" discoveredthat through a simple treatment of gray castiron melts, e. g, in a ladle, the mode of occurrence of the graphiteflakes in gray cast iron can be materially ailected in a. beneficialmanner to improve markedly the properties of the cast iron.

It is an object of the present invention to provide a gray-cast ironcontaining graphite having a compacted flake form, and a method ofproducing the same.

It is a further object of the present invention to provide a gray castiron having an improved combination of founding properties together withimproved mechanical and physical properties.

It is a still further object of the present invention to provide a graycast iron having an improved combination of mechanical and physicalproperties even in the presence of relatively high carbon contents.

Other objects and advantages of the present invention will becomeapparent to those skilled,

in the art from the following description, taken in conjunction with thedrawings in which:

Figure 1 is a reproduction of a photomicrograph taken at 100magnifications showing the structure of a plain gray cast iron devoid ofthe special element contemplated by the present invention;

Fig. 2 is a reproduction of a photomicrograph taken at IOOmagnificationsand depicting the structure of the same gray cast iron as shown in Fig.l but containing the special element in an amount within the rangecontemplated by the present invention;

Fig. 3 is a reproduction of a photomicrograph taken at 100magnifications showing the structure of another plain gray cast irondevoid of the special element contemplated by the present invention; and

Fig. 4 is a reproduction of a. photomicrograph taken at 100magnifications and depicting the effect of the special elementcontemplated by the present invention upon the structure of the graycast iron shown in Fig. 3.

Broadly stated, the present invention provides a gray cast ironcontaining at least about 50% iron, carbon and silicon within the castiron range and containing a controlled amount of retained magnesiumwithin the range of from at least about 0.02% up to 0.04%. The productof the invention is characterized by a microstructure in which thegraphite appears in the form of compacted flakes, i. e., the flakegraphite in the alloy of the invention appears as particles which areshorter and thicker than the flakes which occur in gray cast ironshaving a similar composition but which are devoid of magnesium.Magnesium has a progressive efiect on the size and form of the graphite.Small quantities within the ranges contemplated,- by the presentinvention shorten the flakes,- while larger quantities of the order ofabout 0.03% promote further reduction in the length of the flakesbesides tending to curl and thicken them. With 0.035% and moremagnesium, there is considerable compacting of the graphite in the formof thick short flakes, irregular masses, some more rounded masses andpossebly occasional spheroids. At least about. 75% of the graphitepresent in the gray cast iron of the invention will be present in thecompacted flake form. V

The magnesium-free base composition of the gray cast iron of theinvention is one which would be a gray cast iron if cast in aninoculated condition. Thus, the product of the invention can A have anymatrix structure found in gray cast iron. For example, a matrix may bepearlite,

ferrite, martensite, austenite, an acicular constituent (e. g., bainiteor other transformation products of austenite explained by the S-curve),tempered martensite or sorbite, etc., or the known combinations thereof.The application of the invention to high strength acicular ironsincreases the strength, toughness, modulus and fatigue resistance. Ingeneral, the process is applicable to any cast iron containing flakegraphite. The influence of composition upon the structural constituentsand properties of the matrix and the control of these structuralconstituents and properties of the matrix are known to those skilled inthe art of gray cast iron.

A feature of the present invention in obtaining the novel gray cast ironproduct containing the shorter, thicker or compacted graphite flakes andhaving high properties is the introduction of magnesium into the moltenbath from which the gray cast iron is made and the retention in thefinal product of amounts of magnesium within the ranges set forth'herein. It is not sufficient merely to add magnesium to the moltenbath. The presence of retained magnesium in the gray cast iron of theinvention is essential in order to obtain the improved properties whichcharacterize the magnesium-containing gray cast iron' product. It hasbeen found that magnesium has a powerful whitening effect on gray castiron. Thus, a gray cast iron melt containing mag nesium in accordancewith the invention will usually produce castings having a carbidicnetwork structure unless inoculation is employed as contemplated by theinvention. It has been found than carbidic network structures must beavoided because they have a deleterious effect upon the propertiesobtainable in-alloys of the invention. However, where special propertiessuch as wear resistance, heat resistance, etc., are desired, primarycarbides may be present. Thus, cast irons containing chromium for heatresistance are improved by the present invention. This is believed to bedue to the fact that oxide penetration along the raphite particles isreduced as compared to the oxide penetration that occurs in ordinarygray cast irons having longer flakes. Likewise, cast irons such as thenickel"- chromium cast irons used for dies in forming sheet metal areimproved by the present invention. The improved properties of these castirons are attributed to their greater modulus of elasticity,

strength and hardness when produced in accordance with the presentinvention. The gray cast irons produced in accordance with the inventionare especially useful in heavy sections where the graphite flakesnormally tend to be quite large when made in a conventional manner.

A graphitizing inoculation of the magnesium containing melt shortlybefore casting is an important feature of the invention and effectivelyprevents the formation of the aforementioned harmful carbide networkstructures. The graphitizing inoculation is preferably made shortlyafter the magnesium introduction, but

may be made simultaneously therewith, and

and 1.2 The magnesium-containing inoculated bath should be cast veryquickly after the inoculation, for example, within about 3 minutes afterinoculation, as it has been found that magnesium is lost if the bath isheld for a substantially longer time and that the inoculating eflectwears off and is lost if the bath is held much longer after inoculation.This can be compensated for by another inoculating addition which mayincorporate a smaller amount of the inoculant, e. g., as little as about0.1% or 0.15% silicon may be sufficient. Silicon may be introduced inthe form of fcrro-silicon, e. g, an iron alloy con-- .taining about 50%to'about 95% silicon, although other metallic silicon-containing agentsor alloys such as nickel-silicon alloys or' nickel silicide,calcium-silicon alloys or calcium silicide, silicon metal, and variousproprietary inoculating alloys commonly used for reducing dendriticismand chill in foundry gray cast irons may be employed.

Aluminum and its alloys are not as preferred as the silicon-containininoculating agents.

As indicated hereinbefore, the magnesiumcontaining gray cast ironcontemplated by the invention can be produced from any molten bath whichwould be a gray cast-iron when cast in an inoculated condition. Thisincludes molten baths having such high graphitizing power that theywould be gray cast irons when solidified regardless of whether or notthey were inoculated. Satisfactory results have been obtained frommolten to 4%, in combination with silicon within the aforementionedrange, preferably within .the range of 1%. to 4.7 and more preferably1.2 to 4.2%.

The magnesium-containing improved gray cast iron of the invention willgenerally contain over 1.7% to less than about 5% carbon. Morepreferably, the carbon content will fall within the range of about 2% toabout 4.5%, especially about 2.5% to about 4% carbon. As will be obviousto those skilled in the art, some of the carbon present in the cast ironis usually required to produce the matrix structure. For example, if thematrix is pearlite, the cast iron will contain about 0.8% combinedcarbon. Other microconstituents of the matrix are also usually deemed tocontain combined carbon, and the combined carbon present in the castiron of the inventionwill usually be within the range of about 0.3% to1.2% when the cast iron is in the as-cast condition. The excess carbonnot required to produce the matrix will be mainly uncombined carbon. Aspointed out hereinbefore, over of the '-un-- combined carbon will be.present in the compacted form of graphite described hereinbefore.

The silicon content of preferred gray cast irons produced in accordancewith the invention will be at least 1% of the cast iron and willusually.

fall within the range of 1.3% to about 5%, preferably within the rangeof 1.5% to- 4.5%. An important effect attributable to magnesium ,in'

gray cast irons produced in accordance with the invention is that ofcausing the graphite to occur in the compacted forms describedhereinbefore. The retained magnesium content of the cast iron willusually be within the range of 0.02% to 0.04%. It is preferred that themagnesium con- .tent be relatively high when the carbon content isrelatively high, i. e., exceed about 0.03% as the carbon content reaches3% or more. Thus, when the carbon content is about 3% a magnesiumcontent of about 0.035% produces good results, when the carbon contentis 3.5% a magnesium content of about 0.039% is satisfactory, and whenthe carbon content is about 4% a magnesium content of about 0.043% ishighly effective. None of the common alloying elements employed in graycast irons, with the possible exception of large amounts of copper, havebeen found to prevent the results of the invention from being obtained.Thus, the gray cast iron may contain usual amounts of alloying elementssuch as nickel, molybdenum, chromium, manganese, aluminum, etc. Nickelmay be present in amounts up to about 40%. As those skilled in the artknow, nickel has an important influence on the matrix structure; forexample, when nickel is present in amounts of about 2%, the matrix willbe pearlitic, when nickel is present in amounts of about 5% the matrixwill be martensitic, and when the nickel content exceeds about 20% thematrix will be austenitic. Chromium may be present in amounts up toabout 3.5%, e. g., 0.01% to 3.5%, and manganese in amounts up to about2.5%, e. g., 0.01% to 2.5%. Preferably, the manganese content is withinthe range of about 0.4 to 0.9%. Molybdenum may be present 'in amounts upto about 2%, e. g., 0.01% .to 2%. It is preferred that copper not bepresent in amounts exceeding about 3%, e. g., 0.01% to 3%. Certainelements, including tin, lead, antimony, bismuth, arsenic, selenium,tellurium, etc., have been found to be subversive to the eifect ofmagnesium in controlling the form of flake graphite in the cast iron,and it is preferred that these elements be avoided although smallamounts, preferably less than about 0.1%, may be tolerated in someinstances. It is preferred that phosphorus (usually considered animpurity) be low, for example, below about 0.25% and more preferablybelow about 0.15%, although amounts as high as 0.5% may be present ifhigh properties, especially impact properties, are not the primaryconsideration. The sulfur content of cast irons made in accordance withthe invention is usually low, i. e., below'about 0.08%. The sulfurcontent depends upon the original sulfur content of the base irons andthe amount of retained magnesium. When 0.03% or more magnesium isretained, the sulfur lever is usually less than about 0.02%, regardlessof the initial quantity in the base iron. With less retained magnesium,the sulfur level may be as high as 0.08% and, in general, the finalsulfur content will vary inversely with the amount of retainedmagnesium. The foregoing applies to base irons with normal sulfurcontents of the order of 0.10% to 0.14%; obviously, in low sulfur baseirons, no relation exists between the sulfur content and the retainedmagnesium. In general, the sulfur content will usually be within therange of 0.015% to 0.08%. The balance of the composition is iron(including small amounts of impurities, preferably less than a total ofabout 0.5%). The iron content is generally at least 50% or more, usuallyat least 55% by weight of the alloy, and in the case of unalloyed orlow-alloyed products will be at least about or 87% of the totalcomposition.

Figures 1 to 4 illustrate the influence of magnesium in shorteningand/or compacting the graphite flakes. Fig. 1 depicts the structure of amagnesium-free cast iron containing about 3.5% carbon and about 2.25%silicon, While Fig. 2 shows the microstructure of the same gray castiron after the introduction of 0.039% magnesium. The effect of magnesiumupon the shape and size of the graphite particles is apparent from acomparison of the two microstructures. Similarly, Figs; 3 and 4 show therespective microstructures of a magnesium-free and amagnesium-containing gray cast iron comprised ofv about 3% carbon and1.75% silicon. The magnesium-containing gray cast iron had a retainedmagnesium content of 0.033%. Again, the effect of magnesium upon thesize and shape of the graphite flakes is apv parent from thesestructures.

For general purposes, it is preferred that products having a pearliticmatrix be employed; and for this purpose, it is preferred to employ acomposition containing the amounts of elements set forth in Table I.

Table I Element Range Per cent Carbon 2.5 to 3.2

Silicon 1.6 to 2.5

Magnesium. 0.02 to 0.04

Nickel 0 to 3 Manganese 0.4 to 0.9

mechanical and physical properties than the cor respondingmagnesium-free base composition.

Magnesium-containing cast irons having a pearlitic matrix andcompositions Within the ranges set forth in Table I will usually bestronger than similar cast irons devoid of magnesium, e. g., themagnesium-containing cast irons will have tensile strengths at leastabout 5000 and up to 30,000 pounds per square inch higher than acomparable magnesium-free-gray cast iron. In addition, themagnesium-containing cast irons will usually have improved ductility,impact resistance, modulus of elasticity and fatigue properties incombination with this greater strength. The properties ofmagnesium-containing gray cast irons having compositions within theranges set forth in Table I will usually be within the ranges set forthin Table II.

Table II Transverse properties: 1

Defi., inches 0.15 to 0.20 Load, pounds 4,500 to 7,500 Tensile strength,p. s. i 45,000 to 75,000 Brinell hardness number 200 to 300 Impactresistance, ft.-lbs 35 to 65 Determined on 1.2inch diameter arbitrationbar over- 12-inch span.

The influence of magnesium upon the properties and structure isparticularly notable in gray cast irons having carbon, silicon andmagnesium contents within the ranges set forth in Table I, and

7 small increases in the percentage of retained magnesium within saidrange have a marked I .eflecta pon the properties, especially thestrength.

The properties-of magnesium-containing cast iron, compositions havingmatrices other than pearlitic matrices will also be higher than theproperties of similar cast iron compositions devoid of magnesium. .Forexample, it can be said that cast iron compositions containing amountsof magnesium as defined by the present invention will ,have tensileproperties at least 3000 pounds per square inch greater than similarmagnesium- 'free. gray cast irons, and this increase in tensile strengthwill usually be accompanied by an increase in impact, ductility, heatresistance and compressive strength. Usually, the improvement will be atleast 5000 pounds per square inch in tensile strength for matrices otherthan austenitic ones where the minimum improvement may be less, e. g.,3000 pounds per squareinch. In order toinsure the high order of improvedproperties in cast irons made in accordance with the invention, it isessential that the cast irons contain amounts of retained magnesium asset forth herein and that these cast irons be substantially devoid ofany carbide network structure, for example, the network that can be seenwith the nakedeye in the fractures of cast irons, such asmagnesium-containing cast irons which have not been effectivelyinoculated.

The necessity for having the required retained magnesium content in theas-cast product made in accordance with the present invention isillustrated by the data in Table III setting forth the mechanicalproperties of similar iron-base compositions devoid of magnesium andcontaining various amounts of magnesium. The properties set forth inTable III are the properties of the product in the as-cast conditiondetermined from standard unmachined arbitration bars having a diameterof 1.2 inches, except for the tensile strength which was determined froma tensile specimen machined out of the arbitration bar. All cast irons,except No. 4 and No. 5, were inoculated with a ladle addition of 0.5%silicon as ferrosilicon.

magnesiu'm addition and shortly before casting.

Table III lComposition: 3.1% C; 1.6% Si: 0.8% Ni; 0.7% Mn; 0.02% P.]

Transverse No. 333 1 'r. s BHN Impact Defl. Load Mg introduced as 80%nickel-20% magnesium alloy. De .=Deflection in inches in transverse testdetermmed on arbitration bar over 12-inch span.

Load: Pounds required to fracture arbitration bar transversely over12-inch span.

'1. S.=Iensiie strength in pounds per square inch.

BH N= Brinell hardness number.

'Im act= Foot pounds required to break full-sized unnotched stan ardarbitration bar in Izod (120 it.-lb.) impact testing machine.

The magnesium-containing cast irons 'Nos. 2 and 3 were inoculatedshortly after the the properties of No. 2. is most No. :5 had a d finitecarb network which was apparent upon inspection of the fractured testbar with the-naked eye; or a low -poweredhand lens. This cast iron,although of the samehardof the composition. of the gray cast irons setforth I in Table IV was iron except for small amounts of impurities. Thetransverse properties and impact properties were determined fromunmachined standard arbitration bars having a diameter of 1.2 inches.and they tensile properties were determined onmachined test specimens.

Table IV NO Per cent Per om Per at. Percent Per cent Per cent C Si Mg-Mn i P Table V Transverse Props. Improve- No. BHN Impact '1. 5. D iven;in

Bed. Load nl'rrlipggangaitglonvlelrerimparable magnesium free gray castiron. Tables IV and V illustrate the application of the invention to;cast irons of varying composition over a wide range of carbon contents.data illustrate the improved properties obtained in accordance with theinvention. A feature of the invention is that cast irons ordinarilconsidered to be soft, weak, gray irons can be markedly improved byasimple ladle-treatment with magnesium and an inoculant while the castiron is in the molten condition.

As pointed out hereinbefore, it is essential that amounts of magnesiumwithin the ranges defined herein be present in the novel product of theinvention. If any undesirable elements which tend to combine with and/orcounteract the effect of magnesium are present in the molten'cast ironbath from which the magnesium-containing product of the invention is tobe produced, the amount of magnesium introduced into the bath should beincreased by the amount required tocounteract the eifect of the presenceof such elements or impurities by removing the elements or by otherwiseovercoming their effects. Sulfur is the magnesium-counteracting elementwhich likely to be present, and when sulfur is These nesium contentrequired by the invention.

present in the molten bath, it is necessary to introduce into the bathan amount of magnesium which is sufl'icient not only to produce thedesired retained magnesium content but also to react with sulfur. Manybaths that can be treated in accordance with the invention will containamounts of sulfur as high as even 0.3% or more. It is thereforenecessary to add an amount of magnesium which is sufilcient to introducemagnesium to combine with this sulfur and to provide an excesssuflicient to give the retained mag- The introduction of about threeparts by Weight of magnesium is required to react with about four partsby weight of sulfur. In actual practice, it is preferred to introduceone part by weight of magnesium for each part by weight of sulfur to beremoved.

The introduction of the essential amounts of magnesium required by thepresent invention can be accomplished in a number of ways. The amount ofmagnesium to be added to the bath will depend upon a number of factorsincluding the retained magnesium content desired, the additional amountsof magnesium required to overcome the presence of interfering elementssuch as sulfur, etc., the amount of magnesium lost by delaying thecasting of the bath after the introduction ofmagnesium,- and theproportion of magnesium recovered in the bath from the magnesiumaddition agent. This last factor presents considerable difiiculties, asit has been found that in many cases no magnesium can be recovered fromthe addition agent employed or only a small amount recovered, e. g., 3%of the amount added. The art has taught that magnesium does not alloy.With iron, and as a matter of fact, when it has been attempted tointroduce metallic magnesium in elemental form into a molten bath ofiron when the latter was at the ordinary elevated temperature requiredfor satisfactory casting, a reaction of such explosive violence tookplace that the molten iron was blown from the receptacle in which it washeld. In addition it is known that the temperatures of molten iron bathsusually exceed the boiling temperature of magnesium. The fact that theintroduction of elemental magnesium into molten iron baths produces areaction of explosive violence has been well recognized in the artheretofore, and the introduction of magnesium .into molten iron has beengenerally regarded as being impossible on a practical scale. It ispreferred to add the magnesium as a metallic agent, such as an alloycontaining about 2% to about 40% magnesium, the balance preferably beingnickel and/or copper. It is more preferred to employ a nickel-base alloycontaining the foregoing amounts of magnesium, especially 4% to 20%magnesium. A detailed description of suitable addition alloys which havebeen employed in producing the magnesium-containing cast iron productsof the present invention can be found in our corresponding United Statespatent application, Serial No. 787,420.

The product containing compacted flake graphite in the as-cast conditionhas been satisfactorily produced by a method which comprisesestablishing a molten bath of such composition that it would be a graycast iron if inoculated (including those cast irons which would be grayeven though not inoculated) and cast in a sand mold or in the mold inwhich it is to be cast, regulating the temperature of the molten bath toa proper casting temperature, e. g., a temperature Within the range ofabout 2450" to 2850 F., but preferably within the range of 2650 to 2750F., transferring all or part of the molten metal of the bath to a ladle,adding a nickel-rich magnesium alloy to the molten metal in the ladle inan amount sufficient to provide a retained magnesium content in thesolidified metal of about 0.02% to 0.04%, inoculating themagnesium-containing molten metal with at least about 0.3%, e. g., 0.4%to 1.2%, of silicon, preferably as ferrosilicon, and then quicklycasting the inoculated magnesium-containing molten metal into molds,preferably within three minutes after the last inoculation. The treatedinoculated metal can be cast in accordance with accepted foundrytechnique for gray cast iron.

An unusual feature of the invention is that the magnesium treatment veryeffectively removes sulfur from the molten ferrous bath even when it isunder the influence of acidic conditions such as created by furnacelinings, ladle linings, slags, etc., of a siliceous nature or otheracidic nature as Well as under neutral or basic conditions created bythe furnace lining, the ladle lining, the slag, etc. Another unusualfeature of the invention is that the removal of sulfur by the magnesiumtreatment does not require the presence of any slag and takes placeregardless of whether a slag is or is not present. For example, sulfurcan be removed by the magnesium treatment from a molten ferrous bathwhile it is not covered by a slag and while it is being held in anacid-lined ladle or other acidlined container.

As indicated hereinbefore, alloys which are cast without inoculation orwhich have been ineffectively inoculated, may contain a carbide networkstructure'which prevents the realization of improved properties.However, properties of such improperly made castings can be improved bya heat treatment above the temperature at which the alpha-gammatransformation takes place in the alloy. The treating time, attemperatures above the critical temperature, will generally be at leastabout one hour but less than about 15 hours, e. g., about 3 to 5 hours.A suitable treatment comprises subjecting a casting to temperaturesbetween about 1750 and 1500 F., although temperatures within the rangeof about 1800" and 1400 F. may be used.

The present invention may be applied to the manufacture of a widevariety of ferrous products which will be apparent to those skilled inthe art from the properties and structure of the ferrous alloy providedby the invention. The cast iron provided by the invention isparticularly use ful where so-called high-quality cast irons of theprior art have been employed. Illustrative products and articles includecrank shafts; stamping dies; heatand oxidation-resistant parts such asfurnace parts, stove parts, grate bars, etc.; machine tool beds andframes, particularly those havingheavy sections; frames for othermachinery, including punching presses etc.; railroad and farm machinerycastings; internal combustion engine castings; etc.

Although the present invention has been described in conjunction with.preferred embodiments, it is'understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Variations and modifications apparent to those skilled inthe art are considered to be within the purview and scope of theinvention and appended claims.

We claim:

1. As an article of manufacture, a casting 11 comprised of a gray castiron containing about 2.5% to 4% carbon, about'l.5% to 4.5% silicon,about 0.01% to 2.5% manganese, at least 0.02% and up to 0.04% magnesium,and the balance essentially iron, said gray cast iron beingcharacterized by a microstructure containing at least 75% of thegraphite in a compacted flake form.

2 As an article of manufacture, a casting containing 0.02% to 0.04%magnesium, the balance being a gray cast iron composition.

3. A gray cast iron containing about 2% to 4.5% carbon, about 1.3% tosilicon, at least about 0.02% and up to 0.04% magnesium, and the balanceessentially all iron and alloying elements.

4. A gray cast iron containing about 1.7% to about 5% carbon, less than1.2% carbon being in the combined form, about 1% to 6% silicon, at leastabout 0.02% and. up to 0.04% magnesium, up to 40% nickel, up to 3.5%chromium, up to 2.5% manganese, up to 2% molybdenum, up to 3% copperandthe balance essentially all iron.

5. A method for producing an improved gray cast iron which comprisesestablishing a bath of molten metal containing at least about 87% iron,1.7% to 5% carbon and 0.5% to 5.5% silicon and having such acomposition'as to be a gray cast iron' when inoculated and cast,introducing into said bath an amount of magnesium suiiicient to providea retained magnesium content in castings made from said bath of at least0.02% and up to 0.04% magnesium, inoculating metal in the bathcontaining said amount of magnesium with at least about 0.3% of siliconand casting the inoculated metal to obtain a gray cast iron containingthe aforesaid amounts of retained magnesium, at least about 87% iron, 1%to 6% silicon and 1.7% to 5% carbon.

6. A method for producing an improved gray cast iron which comprisesestablishing a bath of molten metal having such a composition as to be agray cast iron when inoculated and cast, introducing into said bath anamount of ma nesium suflicient to provide av retained magnesium contentin castings made from said bath of at least 0.02% and up to 0.04%magnesium, inoculating metal in the bath containing said amount ofmagnesium in such amanner that the inoculation does not precede themagnesium introduction, and casting the inoculated metal to obtain agray cast iron containing the aforesaid amounts of retained magnesium.

7. A casting having a microstructure containing in the as-cast conditionuncombined carbon in the form of compacted particles, said castingcomprising about 0.03% to about 0.04% magnesium with the balance a graycast iron composition containing at least 87% iron and being devoid ofsubversive amounts of elements materially interfering with theoccurrence of the aforesaid form of uncombined carbon.

8. As a new article of manufacture, an iron casting characterized by amicrostructure containing compacted particles of uncombined carbon in amatrix of the group consisting of pearllte and ferrite and containingabout 0.02% to about 0.04% magnesium to promote the occurrence ofuncombined carbon as compacted particles with the balance a gray castiron composition normally containing uncombined carbon in the form ofelongated flakes in the matrix,

said gray cast iron composition being devoid of subversive amounts ofelements which materially interfere with the aforesaid occurrence, of

uncombined carbon.

KEITH DWIGHT MILIIS. ALBERT PAUL GAGNEBIN. NORMAN BODEN FILLING.

REFERENCES orran The following references are of rccordlll the m of thispatent:

UNITED STATES PATENTS Name I Date Meehan Sept. 4, 1928 OTHER REFERENCESNumber

